The present invention relates to low density, high strength carbon foams prepared by the controlled foaming of petroleum pitch and coal particulate blends.
ASTM standards DD5515-97, xe2x80x9cStandard Test Method for the Determination of Swelling Properties of Bituminous Coalxe2x80x9d and D720-91 xe2x80x9cStandard Test Method for Free Swelling Index of Coalxe2x80x9d both define conditions for measuring the inherent property of coals to xe2x80x9cswellxe2x80x9d upon heating in an uncontrolled combustion situation. Hence, the propensity of coal to swell is well known in the prior art. To the best of our knowledge, however, no one has attempted to take advantage of this property of coals to swell by controllably xe2x80x9cswellingxe2x80x9d a coal product to obtain a highly useful, low density, porous carbon product.
U.S. patent application Ser. No. 09/902,828 filed Jul. 7, 2001 describes coal-based cellular or porous products having a density of preferably between about 0.1 g/cm3 and about 0.8 g/cm3 that are produced by the controlled heating of coal particulate preferably up to xc2xc inch in diameter in a xe2x80x9cmoldxe2x80x9d and under a non-oxidizing atmosphere. According to a specifically preferred embodiment, the starting material coal has a free swell index as determined by aforementioned ASTM D720 test of between about 3.5 and about 5.0. The porous product thereby produced, preferably as a net shape or near net shape, can be machined, adhered and otherwise fabricated to produce a wide variety of low cost, low density products, or used in its preformed shape as a filter, heat or electrical insulator etc. Such cellular products, without further treatment and/or the addition of strengthening additives have been shown to exhibit compressive strengths of up to about 4000 psi. Impregnation with appropriate materials or the incorporation of various strength improving additives can further increase the compressive, tensile and other properties of these cellular materials. Further treatment by carbonization or graphitization yields cellular products that can be used as electrical or heat conductors.
A variety of carbon foams have been produced by the foaming of petroleum pitch largely through the use of blowing or foaming agents. While these materials, carbon foams exhibit higher thermal conductivities than coal-based carbon foams produced as described in the aforementioned U.S. Patent Application, they are considerably weaker and hence not well adapted to application in areas where such physical properties as compressive, tensile and shear strength as well as impact resistance are of significant importance.
Thus, while the methods and products described in the foregoing U.S. Patent Application and those produced by certain competitive methods from petroleum pitch are entirely satisfactory for certain defined purposes that either demand high strength in exchange for high thermal conductivity or vice vesa, it has been deemed desirable to provide foams of yet lower density, i.e. on the order of or below about 0.4 g/cm3, of a wider variation in cell size and of increased thermal conductivity that exhibit strengths approaching those of the coal-based carbon foams previously described. Additionally, the ability to better tailor the foregoing properties to meet specific requirements is also highly desirable.
It is therefore an object of the present invention to provide carbon foams produced from starting materials that permit better control of and variation in such properties as foam density, larger cell size range and enhanced thermal conductivity to meet specific end use requirements.
It is another object of the present invention to provide such materials and methods that utilize starting materials at least partially different than those described in the foregoing U.S. patent application Ser. No. 09/902,828.
According to the present invention, carbon foams are produced largely in accordance with the methods described in foregoing U.S. patent application Ser. No. 09/902,828, but with starting materials that comprise from about 10 to about 90% by weight of ground petroleum pitch and from about 90 to about 10% by weight of bituminous coal particulate exhibiting a free swell index of from about 3.5 to about 5.0. The relatively lower melting point of the petroleum coke permits this material to flow about the coal particulate thereby encapsulating the coal particulate prior to the onset of the foaming of the coal particulate thereby permitting the production of carbon foams that exhibit larger bubble or cell sizes and consequently are of lower density than those conventionally produced in accordance with solely coal particulate starting materials. The presence of the petroleum pitch permits the fabrication of carbon foams exhibiting enhanced thermal conductivity. The blending of varying relative amounts of petroleum pitch and coal particulate in the carbon foam production process permits the formulation of custom carbon foams that exhibit the appropriate balances between strength, as provided by the coal particulate, and thermal conductivity, as provided by the petroleum pitch in the starting material.